Films for X-ray photography that are utilized in medical diagnoses are commonly used in such a way that a film and a fluorescent intensifying screen are used in combination at the time a photograph is taken. This is because X-rays having passed through a front-side intensifying screen are further utilized on a back-side intensifying screen, taking account of the influence of X-rays on human bodies and for the purpose of effectively utilizing X-rays.
In recent years, under circumstances where films and intensifying screens are increasingly made to have a higher sensitivity, there is an increasing demand for a higher image quality of images for medical diagnoses. In particular, with regard to sharpness, it is strongly sought to improve performance so that the state of nidi or affected parts can be examined in more detail.
In light-sensitive silver halide X-ray photographic materials comprising a support coated with emulsions on its both sides, however, the light emitted from one intensifying screen passes through its adjoining silver halide emulsion layer, and the light thus having passed is scattered through the support to cause a phenomenon in which what is called cross-over exposure, which is a phenomenon of imagewise exposing the silver halide emulsion layer on the opposite side with the scattered light, occurs from both sides. This is a great factor that brings about a deterioration of the sharpness of an image.
A number of proposals have been hitherto made so that the cross-over exposure occurring from the both sides can be decreased and the sharpness can be improved. For example, Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) No. 132945/1986, British Patent No. 821,352, etc. disclose that a dye is used in a silver halide emulsion layer or in another component layer.
With approaches from the direction of films, approaches to improvement are also made from the direction of intensifying screens. For example, Japanese Patent O.P.I. Publication No. 110538/1990 also discloses that silver halide emulsion layers having different color sensitivities are provided on the side-A and side-B, and fluorescent intensifying screens having emission spectra that are respectively in agreement with their color sensitivities are used.
These techniques for improvement, however, can not avoid the deterioration of sharpness so far as emulsions are coated on both sides and two sheets of intensifying screens are used. In particular, in oblique incidence photographing in which X-rays are made obliquely incident on the film surface in X-ray photographing systems, any disalignment between images on both sides results in a serious lowering of sharpness.
In order to make sharpness higher, it has been also attempted to provide an emulsion layer only on one side and carry out exposure only from one side. In this method, however, the sensitivity is so low that an attempt to make sensitivity higher on one side necessarily causes an increase in layer thickness of a light-sensitive emulsion layer, which brings about the disadvantages that the fixing performance, washing performance, drying speed, etc. on the one side are lowered, and images are deteriorated due to color remaining of sensitizing dye, residual silver or residual hypo. Hence, this method is not preferable.
The present inventors have ever made various studies on a method in which, in a light-sensitive silver halide photographic material having silver halide emulsion layers on both sides, the sensitivities and coating silver weights are made different between an emulsion layer on one side (side-A) and an emulsion layer on the other side (side-B) and a fluorescent intensifying screen is used only on one side so that the sharpness can be improved. As a result, they found that the surface properties of a film were not balanced between the side-A and side-B because of the side-A and side-B constituted of emulsions, silver weights, binder weights, etc. different from each other.
For example, on the side-A and side-B, a difference in the weights of binders mainly composed of gelatin in the both or a difference in silver weight caused a difference in tensile strength in the film, which brought about undesirable phenomenons such that curling occurred and an insufficient adjustment of water content in both sides caused blocking or sticking of films.
In recent years, the photographing system for X-ray films has been automated, and hence automatic transport performance of films is considered to be one of important performances. Under such circumstances, curling or sticking of films can be a fatal defect, and hence an immediate improvement has been sought.
Moreover, it is very difficult to readily distinguish in a darkroom the obverse or reverse of a film having emulsion layers on its both sides, to carry out operation, and it is usual to make a mistake in a high probability.
Thus, this is even one of the reasons why the films having differences in performances between the obverse and reverse have not been hitherto used. Accordingly, it has been pressingly needed to establish a system in which photographs can be taken without a mistake to reversely use the sides of a film, even when films having differences in performances between the obverse and reverse are used.
The present inventors have proposed a light-sensitive silver halide X-ray photographic material, and an image forming method, that can solve such technical problems, that can be a light-sensitive material with a high sensitivity and a superior sharpness, and that can obtain a high image quality even in oblique incidence photographing, can give a good film transport performance, and can obtain an image with a high sharpness where a fluorescent intensifying screen is used only on one side and exposure is carried out only from one side, as disclosed in Japanese Patent Application No. 332970/1990. In the invention disclosed therein, emulsion layers with respectively different photographic performances are provided on both sides of a transparent support, and exposure is carried out from the side of higher sensitivity, whereby an X-ray image with a high sharpness can be obtained without any inferiority in sensitivity, to conventional light-sensitive materials comprising a support provided on both sides thereof with emulsion layers having the same photographic performances. This method can dramatically improve the sharpness, but on the other hand it was found that in X-ray photography the quantum mottles tend to be conspicuous at a low-density portion, resulting in a deterioration of graininess.
In the case of photographs where importance is attached to the diagnostic performance at a low-density portion and also a portion with less difference in image density, as in chest X-ray photographs, a lowering of diagnostic performance which accompanies the deterioration of graininess tends to surpass an improvement of diagnostic performance which attributes an improvement in sharpness.